The subject invention relates to improving the electrical efficiency of amplifiers and particularly of power amplifiers providing output signals having a plurality of different power levels.
Many types of modern electronic equipment employ power amplifiers. More specifically, radio frequency (RF) power amplifiers are utilized in transmitters to increase the power level of amplitude modulated (AM) signals, for example, wherein the magnitude of a fixed frequency carrier signal is continuously varied in accordance with the instantaneous magnitude of a lower frequency modulating signal. The relative magnitudes of the modulating signal and the carrier signal define a percentage of AM. 100% AM occurs when the magnitude of the modulating signal causes the magnitude of the carrier to vary from zero to twice the quiescent magnitude of the carrier signal. Such power amplifiers must be designed to deliver the peak envelope power (PEP) which occurs at 100% AM.
Power amplifiers are also used to increase the power level of frequency modulated (FM) signals wherein the amount of frequency deviation of a carrier signal varies with the amplitude of a modulating signal. The power level of a FM signal doesn't vary with the amplitude of the modulating signal but does vary with the carrier amplitude.
Design of power amplifiers typically involves determining the optimum load resistance which provides a predetermined maximum output power with a particular power supply voltage of a constant direct current (DC) magnitude. When such power amplifiers are then operated at less than the predetermined maximum output power, reduced average efficiency occurs. Efficiency can be defined as the RF power out of an amplifier divided by the DC power into the amplifier. The portion of the DC power not changed into RF output power is changed into undesirable heat by the power amplifier. This heat increases the thermal stress on the RF power transistors therein and also results in higher chassis temperatures. The heating of RF power transistors causes metal migration therein which tends to reduce the lifetime of such transistors, for instance. Increased chassis temperature tends to cause undesirable thermally generated effects on other amplifier components which can either require additional compensation circuitry or cause malfunctions.
Moreover, the reduced efficiency undesirably drains the batteries of battery operated equipment such as portable radios. For instance, it is sometimes required to switch FM or phase modulated (PM) or other signal transmitting equipment to different power or carrier levels to preserve batteries or to reduce interference with other spectrum users. These prior art transmitters or amplifiers having power supply voltages of fixed magnitudes tend to operate inefficiently when operated at a variety of output power levels.